1. Field of the Invention
The present invention relates to a magnetometer for measuring a weak magnetic field using a superconducting quantum interference device (hereinafter briefly referred to as SQUID). Specifically, it relates to an improved configuration of a magnetometer in which a high frequency current is fed to a living body, and the resulting change in magnetic field or a nuclear magnetic resonance signal is detected by a pickup coil magnetically or electrically connected to the SQUID. More specifically, it relates of a magnetometer including a normal conducting member as the pickup coil which is arranged outside a cryostat.
2. Description of the Related Arts
In a conventional apparatus for measuring a magnetic field, a pickup coil made of a superconducting member is used, and a SQUID and the pickup coil are both cooled to a superconducting state to thereby detect magnetic field changes with activities of neurons in brain cells (magnetoencephalography) or magnetic field changes with action currents of cardiac muscle cells (magnetocardiography). In this configuration, the pickup coil is inevitably arranged distant from an inspected subject.
Impedance cardiography has been developed in which a high frequency current is fed to a living body, and an electric potential varying with changes in blood volume flowing in the living body is measured in order to monitor changes in electric potential with mechanical motions such as blood flow or the systole and diastole of the heart [Aerospace Medicine; Vol. 37 (1966), pp. 1208-1212 (Reference 1) and Aviation, Space, and Environmental Medicine; Vol. 70, No. 8 (1999), pp. 780-789 (Reference 2)].
In another process, a high frequency current is applied to a living body to thereby measure a magnetic field [Phys. Med. Biol.; Vol. 46, (2001), pp. N45-N48 (Reference 3)]. This process uses a pickup coil placed inside a cryostat.
Japanese Patent Laid-Open No. 6-225860 (1994) (Reference 4) mentions an apparatus for measuring the spatial distribution of electrical impedance as an industrial field of the invention. In the apparatus, a source of electrical current is electrically connected to at least two feed electrodes which impress a feed current from the source in an examination region of a subject to form a current distribution corresponding to electrical impedance distribution and positions of the electrodes. The resulting magnetic field is measured at points outside the examination region, and an equivalent current density distribution is reconstructed within the examination region from the measured values of the magnetic field. The equivalent current density distribution at the measuring points is that which would be generated by a theoretical magnetic field which best coincides with the measured magnetic field caused by the distribution of the current. The invention described in this reference is directed to provide an apparatus for identifying the spatial distribution of electrical impedance in a subject which has a high sensitivity for the magnetic fields generated by the distribution of current in the examination region.
Alternatively, SQUIDs are used to detect a magnetic resonance signal with a high sensitivity [Appl. Phys. Lett.; Vol. 70, No. 8 (1997), pp. 1037-1039 (Reference 5) and Rev. Sci. Instrum.; Vol. 69, No. 3 (1998), pp. 1456-1462 (Reference 6)]. In such an apparatus using SQUIDs, the magnetic resonance signal is detected by a process in which a pickup coil is placed inside a cryostat as in conventional apparatus for measuring a magnetic field in a living body, or by a process in which a sample is placed in the cryostat, and the magnetic resonance signal in the sample is detected at cryogenic temperatures. According to the former process, the pickup coil cannot be sufficiently brought close to the inspected subject and SQUID magnetometer can not be operated because it should be placed in a static magnetic field. According to the latter process, the sample must be cooled to cryogenic temperatures, and the magnetic resonance signal cannot be detected in samples at an ordinary temperature.
Conventional impedance cardiography based on measurement of electric potential requires a large number of electrodes to identify the state of local blood and is not suitable as a general measuring method. A technique has therefore been developed for real-time and non-touch monitoring of a change in magnetic field with mechanical motion such as the blood flow or the systole and diastole of the heart (Reference 3). Such a change in magnetic field with mechanical motion such as the blood flow or the systole and diastole of the heart can be detected by using the conventional superconducting pickup coil placed inside the cryostat. However, according to this technique, the pickup coil cannot sufficiently be brought close to the inspected subject.
The technique described in Reference 4 can detect the distribution in electric impedance generated by the current fed from the feed electrode at a certain time but cannot detect, in real-time, a change in electric impedance with time.
Accordingly, an object of the present invention is to detect a change in magnetic field with a high sensitivity outside a cryostat using a SQUID magnetometer including a pickup coil made of a normal conducting material, which change in magnetic field is induced by mechanical motions such as a blood flow in an organ of a living body.
Another object of the present invention is to provide a SQUID magnetometer using an ordinary-temperature coil which can detect magnetic resonance signals with a high sensitivity even in a low magnetic field and can be brought in intimate contact with an inspected subject at an ordinary temperature.
To achieve the above objects, the present invention provides, in one aspect, an apparatus for measuring a magnetic field (a SQUID magnetometer). The apparatus includes a device for feeding a current to a living body; a pickup coil for detecting a magnetic field induced in the living body by action of the device for feeding a current; a superconducting quantum interference device; and a device for connecting the pickup coil to the superconducting quantum interference device. In the apparatus, the pickup coil is made of a normal conducting member.
In another aspect, the present invention provides an apparatus for measuring a magnetic field. This apparatus includes a device for feeding a current to a subject; a pickup coil for detecting a magnetic field in the subject; a superconducting quantum interference device; a cryostat for holding the superconducting quantum interference device; and a device for connecting the pickup coil to the superconducting quantum interference device. In the apparatus, the pickup coil is made of a normal conducting member and is arranged outside the cryostat.
In addition and advantageously, the present invention provides an apparatus for examination. This apparatus includes a device for applying an alternating current to an inspected subject; a detecting probe for detecting a magnetic field generated from the inspected subject; a superconducting quantum interference device connected to the detecting probe; a cryostat for holding the superconducting quantum interference device; and a detector for extracting a magnetic field with a desired frequency component from the detecting probe by using the alternating current applied to the inspected subject as a reference signal.
In the apparatus for measuring a magnetic field (SQUID magnetometer) of the present invention, a preferred configuration is as follows: Specifically, at least one pickup coil for measuring a magnetic field made of a normal conducting material is arranged outside the cryostat, and at least one SQUID electrically or magnetically connected to the pickup coil is arranged inside the cryostat. A cryogenic cooling medium is charged into the cryostat to thereby hold the SQUID in a superconducting state. At least two electrodes are placed in at least two positions, such as the head and leg, of a subject or at least two positions of a metal conductor. The apparatus includes a driving circuit for driving the SQUID and an oscillator for feeding a high frequency current to the electrodes. The output terminal of the driving circuit is connected to a high-pass filter circuit, a phase-shift detector, a band-pass filter circuit, and an amplifier. The apparatus further includes a device for feeding output signals (hereinafter, the output signal from the amplifier obtained by feeding the high frequency current to the subject is referred to as xe2x80x9cimpedance magnetocardiogram signalxe2x80x9d) from the amplifier to a computer to thereby collect data and for displaying and calculating the collected data. It further includes a coil for applying a compensation magnetic field with an inverse phase in the vicinity of the pickup coil, and a device for optimizing a magnetizing current fed to the compensation coil with an inverse phase based on the current data obtained from a differential amplifier for controlling the high frequency current flowing through the subject or the metal conductor.